It's not definitive that ISIS bombed a Russian charter jet out of the sky over the Sinai Peninsula, but if it did, the incident dramatically raises the stakes for the U.S.-led coalition in its battle against the Islamist terrorist group, security experts said.

Among the likely scenarios if it turns out that ISIS has developed the capability to target planes is that Russia would sharply increase its military presence in Syria, where it says it has declared war on the organization.

Until Wednesday, U.S. officials had said there was no hard evidence that ISIS or another terrorist group brought down the Metrojet-operated Airbus A321 over Egypt on Saturday, killing all 224 people on board.

Officials specifically dismissed claims by ISIS that it shot the plane down with a missile. But ISIS might not have had to use missiles — U.S. officials told NBC News on Wednesday there was significant evidence leading to "great confidence" among government analysts that a bomb loaded before takeoff was the actual mechanism.

In this photo made available Monday, Nov. 2, 2015, and provided by Russian Emergency Situations Ministry, Egyptian Military on cars approach a plane's tail at the wreckage of a passenger jet bound for St. Petersburg in Russia that crashed in Hassana, Egypt, on Sunday, Nov. 1, 2015. The Russian cargo plane on Monday brought the first bodies of Russian victims killed in a plane crash in Egypt home to St. Petersburg, a city awash in grief for its missing residents. (Maxim Grigoriev/Russian Ministry for Emergency Situations via AP)

In this Russian Emergency Situations Ministry photo, made available on Monday, Nov. 2, 2015, Russian Emergency Ministry experts work at the crash site of a Russian passenger plane bound for St. Petersburg in Russia that crashed in Hassana, Egypt's Sinai Peninsula, on Monday, Nov. 2, 2015. A Russian cargo plane on Monday brought the first bodies of Russian victims home to St. Petersburg, from Egypt.(Russian Ministry for Emergency Situations photo via AP)

In this Russian Emergency Situations Ministry photo, made available on Monday, Nov. 2, 2015 Egyptian soldiers collect personal belongings of plane crash victims at the crash site of a passenger plane bound for St. Petersburg in Russia that crashed in Hassana, Egypt's Sinai Peninsula, on Monday, Nov. 2, 2015. A Russian cargo plane on Monday brought the first bodies of Russian victims home to St. Petersburg, from Egypt.(Russian Ministry for Emergency Situations photo via AP)

In this Russian Emergency Situations Ministry photo, made available on Monday, Nov. 2, 2015, Russian Emergency Ministry experts work at the crash site of a Russian passenger plane bound for St. Petersburg in Russia that crashed in Hassana, Egypt's Sinai Peninsula, on Monday, Nov. 2, 2015. A Russian cargo plane on Monday brought the first bodies of Russian victims home to St. Petersburg, from Egypt.(Russian Ministry for Emergency Situations photo via AP)

Debris belonging to the A321 Russian airliner are seen at the site of the crash in Wadi el-Zolmat, a mountainous area in Egypt's Sinai Peninsula on November 1, 2015. International investigators began probing why a Russian airliner carrying 224 people crashed in Egypt's Sinai Peninsula, killing everyone on board, as rescue workers widened their search for missing victims. AFP PHOTO / KHALED DESOUKI (Photo credit should read KHALED DESOUKI/AFP/Getty Images)

Debris of a Russian airplane is seen at the site a day after the passenger jet bound for St. Petersburg, Russia, crashed in Hassana, Egypt, on Sunday, Nov. 1, 2015. The Metrojet plane, bound for St. Petersburg in Russia, crashed 23 minutes after it took off from Egypt's Red Sea resort of Sharm el-Sheikh on Saturday morning. The 224 people on board, all Russian except for four Ukrainians and one Belarusian, died. (AP Photo)

In this photo made available Monday, Nov. 2, 2015, and provided by Russian Emergency Situations Ministry, Egyptian Military experts examine a piece of an engine at the wreckage of a passenger jet bound for St. Petersburg in Russia that crashed in Hassana, Egypt, on Sunday, Nov. 1, 2015. The Russian cargo plane on Monday brought the first bodies of Russian victims killed in a plane crash in Egypt home to St. Petersburg, a city awash in grief for its missing residents. (Maxim Grigoriev/Russian Ministry for Emergency Situations via AP)

A Russian investigator walks near wreckage a day after a passenger jet bound for St. Petersburg, Russia, crashed in Hassana, Egypt, on Sunday, Nov. 1, 2015. The Metrojet plane, bound for St. Petersburg in Russia, crashed 23 minutes after it took off from Egypt's Red Sea resort of Sharm el-Sheikh on Saturday morning. The 224 people on board, all Russian except for four Ukrainians and one Belarusian, died. (AP Photo/Amr Nabil)

A helicopter of the Egyptian security forces fly over the site a day after a passenger jet bound for St. Petersburg, Russia, crashed in Hassana, Egypt, on Sunday, Nov. 1, 2015. The Metrojet plane, bound for St. Petersburg in Russia, crashed 23 minutes after it took off from Egypt's Red Sea resort of Sharm el-Sheikh on Saturday morning. The 224 people on board, all Russian except for four Ukrainians and one Belarusian, died. (AP Photo)

In this photo taken Sunday, Nov. 1, 2015 and provided by Russian Ministry for Emergency Situations Monday, Nov. 2, Russian Emergency Situations Minister Vladimir Puchkov, left, talks with Russian Transport Minister Maxim Sokolov, fifth right, as they inspect the wreckage of a passenger jet bound for St. Petersburg in Russia that crashed in Hassana, Egypt. A Russian cargo plane on Monday brought the first bodies of Russian victims killed in the plane crash in Egypt home to St. Petersburg, a city awash in grief for its missing residents. (Maxim Grigoriev/Russian Ministry for Emergency Situations via AP)

In this photo made available Monday, Nov. 2, 2015, and provided by Russian Emergency Situations Ministry, Egyptian Military on cars approach a plane's tail at the wreckage of a passenger jet bound for St. Petersburg in Russia that crashed in Hassana, Egypt, on Sunday, Nov. 1, 2015. The Russian cargo plane on Monday brought the first bodies of Russian victims killed in a plane crash in Egypt home to St. Petersburg, a city awash in grief for its missing residents. (Maxim Grigoriev/Russian Ministry for Emergency Situations via AP)

SUEZ, EGYPT - NOVEMBER 01: A plane part is seen as the Egyptian officials inspect the crash site of Russian Airliner in Suez, Egypt on November 01, 2015. A Russian Airbus-321 airliner with 224 people aboard crashed in Egypt's Sinai Peninsula on yesterday. According to Egypts Civil Aviation Authority, the plane had been lost contact with air-traffic controllers shortly after taking off from the Egyptian Red Sea resort city of Sharm el-Sheikh en route to St Petersburg. (Photo by Alaa El Kassas/Anadolu Agency/Getty Images)

Debris of the A321 Russian airliner lie on the ground a day after the plane crashed in Wadi al-Zolomat, a mountainous area in Egypt's Sinai Peninsula, on November 1, 2015. International investigators began probing why the Russian airliner carrying 224 people crashed in the Sinai Peninsula, killing everyone on board, as rescue workers widened their search for missing victims. AFP PHOTO / KHALED DESOUKI (Photo credit should read KHALED DESOUKI/AFP/Getty Images)

Russian investigators stand near debris, luggage and personal effects of passengers a day after a passenger jet bound for St. Petersburg in Russia crashed in Hassana, Egypt, on Sunday, Nov. 1, 2015. The Metrojet plane crashed 23 minutes after it took off from Egypt's Red Sea resort of Sharm el-Sheikh on Saturday morning. The 224 people on board, all Russian except for four Ukrainians and one Belarusian, died. (AP Photo/Amr Nabil)

Debris of the A321 Russian airliner lie on the ground a day after the plane crashed in Wadi al-Zolomat, a mountainous area in Egypt's Sinai Peninsula, on November 1, 2015. International investigators began probing why the Russian airliner carrying 224 people crashed in the Sinai Peninsula, killing everyone on board, as rescue workers widened their search for missing victims. AFP PHOTO / KHALED DESOUKI (Photo credit should read KHALED DESOUKI/AFP/Getty Images)

SUEZ, EGYPT - NOVEMBER 01: Egyptian officials inspect the crash site of Russian Airliner in Suez, Egypt on November 01, 2015. A Russian Airbus-321 airliner with 224 people aboard crashed in Egypt's Sinai Peninsula on yesterday. According to Egypts Civil Aviation Authority, the plane had been lost contact with air-traffic controllers shortly after taking off from the Egyptian Red Sea resort city of Sharm el-Sheikh en route to St Petersburg. (Photo by Alaa El Kassas/Anadolu Agency/Getty Images)

SUEZ, EGYPT - NOVEMBER 01: Egyptian officials inspect the crash site of Russian Airliner in Suez, Egypt on November 01, 2015. A Russian Airbus-321 airliner with 224 people aboard crashed in Egypt's Sinai Peninsula on yesterday. According to Egypts Civil Aviation Authority, the plane had been lost contact with air-traffic controllers shortly after taking off from the Egyptian Red Sea resort city of Sharm el-Sheikh en route to St Petersburg. (Photo by Alaa El Kassas/Anadolu Agency/Getty Images)

Members of the Egyptian security forces stand guard at the site a day after a passenger jet bound for St. Petersburg, Russia crashed in Hassana, Egypt, on Sunday, Nov. 1, 2015. The Metrojet plane, bound for St. Petersburg in Russia, crashed 23 minutes after it took off from Egypt's Red Sea resort of Sharm el-Sheikh on Saturday morning. The 224 people on board, all Russian except for four Ukrainians and one Belarusian, died. (AP Photo)

CAIRO, EGYPT - OCTOBER 31: Ambulances bring the bodies of the victims found at crash site of Russian Airliner, at Zinhoum Hospital's Morgue in Cairo, Egypt on October 31, 2015. A Russian Airbus-321 airliner with 224 people aboard crashed in Egypt's Sinai Peninsula on Saturday, according to the Egyptian Prime Minister's office. According to Egypts Civil Aviation Authority, the plane lost contact with air-traffic controllers shortly after taking off from the Egyptian Red Sea resort city of Sharm el-Sheikh en route to St Petersburg. (Photo by Ibrahim Ramadan/Anadolu Agency/Getty Images)

CAIRO, EGYPT - OCTOBER 31: Ambulances bring the bodies of the victims found at crash site of Russian Airliner, at Zinhoum Hospital's Morgue in Cairo, Egypt on October 31, 2015. A Russian Airbus-321 airliner with 224 people aboard crashed in Egypt's Sinai Peninsula on Saturday, according to the Egyptian Prime Minister's office. According to Egypts Civil Aviation Authority, the plane lost contact with air-traffic controllers shortly after taking off from the Egyptian Red Sea resort city of Sharm el-Sheikh en route to St Petersburg. (Photo by Ibrahim Ramadan/Anadolu Agency/Getty Images)

CAIRO, EGYPT - OCTOBER 31: Ambulances bring the bodies of the victims found at crash site of Russian Airliner, at Zinhoum Hospital's Morgue in Cairo, Egypt on October 31, 2015. A Russian Airbus-321 airliner with 224 people aboard crashed in Egypt's Sinai Peninsula on Saturday, according to the Egyptian Prime Minister's office. According to Egypts Civil Aviation Authority, the plane lost contact with air-traffic controllers shortly after taking off from the Egyptian Red Sea resort city of Sharm el-Sheikh en route to St Petersburg. (Photo by Ibrahim Ramadan/Anadolu Agency/Getty Images)

CAIRO, EGYPT - OCTOBER 31: Ambulances bring the bodies of the victims found at crash site of Russian Airliner, at Zinhoum Hospital's Morgue in Cairo, Egypt on October 31, 2015. A Russian Airbus-321 airliner with 224 people aboard crashed in Egypt's Sinai Peninsula on Saturday, according to the Egyptian Prime Minister's office. According to Egypts Civil Aviation Authority, the plane lost contact with air-traffic controllers shortly after taking off from the Egyptian Red Sea resort city of Sharm el-Sheikh en route to St Petersburg. (Photo by Ibrahim Ramadan/Anadolu Agency/Getty Images)

Egyptian ambulance arrives to Kabret military air base by the Suez Canal on October 31, 2015, after victims of a Russian airliner that crashed in the Sinai Peninsula were brought to the base before being transported to a morgue. Egypt's government said 15 bodies have been recovered and transferred to a morgue so far from the site of the crash. AFP PHOTO / KHALED DESOUKI (Photo credit should read KHALED DESOUKI/AFP/Getty Images)

Egyptian paramedics load the corpses of Russian victims of a Russian passenger plane crash in the Sinai Peninsula, into a military plane at Kabret military air base by the Suez Canal on October 31, 2015. Egypt's government said 15 bodies have been recovered and transferred to a morgue so far from the site of the crash. AFP PHOTO / KHALED DESOUKI (Photo credit should read KHALED DESOUKI/AFP/Getty Images)

SUEZ, EGYPT - OCTOBER 31: Egyptian officials receives the corpses of 34 passengers, found at crash site of Russian Airliner, at Kabret Military Base in Suez, Egypt on October 31, 2015. A Russian Airbus-321 airliner with 224 people aboard crashed in Egypt's Sinai Peninsula on Saturday, according to the Egyptian Prime Minister's office. According to Egypts Civil Aviation Authority, the plane lost contact with air-traffic controllers shortly after taking off from the Egyptian Red Sea resort city of Sharm el-Sheikh en route to St Petersburg. (Photo by Mostafa El Shemy/Anadolu Agency/Getty Images)

Egyptian ambulances line up at Kabret military air base by the Suez Canal on October 31, 2015, after victims of a Russian airliner that crashed in the Sinai Peninsula were brought to the base before being transported to a morgue. Egypt's government said 15 bodies have been recovered and transferred to a morgue so far from the site of the crash. AFP PHOTO / KHALED DESOUKI (Photo credit should read KHALED DESOUKI/AFP/Getty Images)

An Egyptian ambulance waits at Kabret military air base by the Suez Canal on October 31, 2015, after victims of a Russian airliner that crashed in the Sinai Peninsula were brought to the base before being transported to a morgue. Egypt's government said 15 bodies have been recovered and transferred to a morgue so far from the site of the crash. AFP PHOTO / KHALED DESOUKI (Photo credit should read KHALED DESOUKI/AFP/Getty Images)

SUEZ, EGYPT - OCTOBER 31: Egyptian officials receives the corpses of 34 passengers, found at crash site of Russian Airliner, at Kabret Military Base in Suez, Egypt on October 31, 2015. A Russian Airbus-321 airliner with 224 people aboard crashed in Egypt's Sinai Peninsula on Saturday, according to the Egyptian Prime Minister's office. According to Egypts Civil Aviation Authority, the plane lost contact with air-traffic controllers shortly after taking off from the Egyptian Red Sea resort city of Sharm el-Sheikh en route to St Petersburg. (Photo by Alaa El Kassas/Anadolu Agency/Getty Images)

Egyptian ambulances carrying the corpses of Russian victims of a Russian passenger plane crash in the Sinai Peninsula, off load the bodies into a military aircraft at Kabret military air base by the Suez Canal on October 31, 2015. Egypt's government said 15 bodies have been recovered and transferred to a morgue so far from the site of the crash. AFP PHOTO / KHALED DESOUKI (Photo credit should read KHALED DESOUKI/AFP/Getty Images)

Egyptian police officers stand guard outside the Zeinhom Morgue in Cairo on October 31, 2015, as they wait for the victims of the Russian charter plane. A Russian plane carrying 224 people crashed in a mountainous part of Egypt's Sinai Peninsula, killing everyone on board, officials said. AFP PHOTO /MOHAMED EL-SHAHED (Photo credit should read MOHAMED EL-SHAHED/AFP/Getty Images)

Egyptian police officers stand guard, next to graffiti condemning police brutality, outside the Zeinhom Morgue in Cairo on October 31, 2015, as they wait for the victims of the Russian charter plane. A Russian plane carrying 224 people crashed in a mountainous part of Egypt's Sinai Peninsula, killing everyone on board, officials said. AFP PHOTO /MOHAMED EL-SHAHED (Photo credit should read MOHAMED EL-SHAHED/AFP/Getty Images)

Egyptian police officers stand guard outside the Zeinhom Morgue in Cairo on October 31, 2015, as they wait for the victims of the Russian charter plane. A Russian plane carrying 224 people crashed in a mountainous part of Egypt's Sinai Peninsula, killing everyone on board, officials said. AFP PHOTO /MOHAMED EL-SHAHED (Photo credit should read MOHAMED EL-SHAHED/AFP/Getty Images)

Egyptian police officers stand guard outside the Zeinhom Morgue in Cairo on October 31, 2015, as they wait for the victims of the Russian charter plane. A Russian plane carrying 224 people crashed in a mountainous part of Egypt's Sinai Peninsula, killing everyone on board, officials said. AFP PHOTO /MOHAMED EL-SHAHED (Photo credit should read MOHAMED EL-SHAHED/AFP/Getty Images)

Egyptian police officers stand guard outside the Zeinhom Morgue in Cairo on October 31, 2015, as they wait for the victims of the Russian charter plane. A Russian plane carrying 224 people crashed in a mountainous part of Egypt's Sinai Peninsula, killing everyone on board, officials said. AFP PHOTO /MOHAMED EL-SHAHED (Photo credit should read MOHAMED EL-SHAHED/AFP/Getty Images)

ALTERNATIVE CROP - In this photo taken on Tuesday, Oct. 20, 2015, The Russian airline Kogalymaviaâs Airbus A321 with a tail number of EI-ETJ on an airstrip of Moscowâs Domodedovo international airport, outside Moscow, Russia. Russia's civil air agency is expected to have a news conference shortly to talk about the Russian Metrojet passenger plane EI-ETJ, that Egyptian authorities say has crashed in Egypt's Sinai peninsula. (AP Photo/Tatiana Belyakova)

At least three top officials at the airport, including its chief of security, were fired Wednesday after serious security lapses were discovered, Egyptian officials told NBC News.

And U.S. officials said investigators are beginning to focus on ISIS operatives or sympathizers as the likely bombers who exploited those weaknesses.

If ISIS was able to plant a bomb on the plane, "it's a real game-changer for the region," demonstrating that at least some of ISIS' grandiose boasts about its capabilities are true, said NBC News national security analyst Kevin Baron, executive editor of security analytics company Defense One.

The United States has already seen evidence that -- even as it has withstood more than 8,000 U.S. and coalition airstrikes -- ISIS has still managed to expand far beyond its base in Syria and Iraq, to Afghanistan, Egypt, Libya and even Nigeria and Yemen.

The group has a great deal of cash and a sophisticated online recruiting structure, making it capable of growing faster than al-Qaeda, which officials said has always been difficult to join. And combined with its "very aggressive" presence in the region, it now has a specific grudge against Russia, which began deploying special operations forces in Syria early last month.

NBC News national security analyst Michael Leiter, who was director of the U.S. National Counterterrorism Center during the administrations of Presidents George W. Bush and Barack Obama, agreed Wednesday not only that "the motive is there" but also that "the capability, I think, is largely there -- even if this is an upgrade."

U.S. officials told NBC News that the powerful state-run Russian media could quickly whip up nationalistic fervor in support of President Vladimir Putin's campaign in Syria.

That makes for a highly volatile situation given Putin's shoot-from-the-hip style, U.S. National Intelligence Director James Clapper said.

Putin sees himself as "a man on white horseback" riding in to save Syria, and he will likely respond by ramping up his forces there, Clapper said Monday at a Defense One summit in Washington, D.C. He said Putin would most likely increase the number of Russian advisers in Syria in the short term.

Regardless of the nature of the response, a senior U.S. military official in the region told NBC News on Wednesday, it's almost certain that Russia will retaliate "heavily and militarily."